The present invention relates to a method for central nervous system (CNS) monitoring, and more specifically to a method of positioning electrodes in an electrode array comprising at least five or at least seven electrodes for monitoring central nervous system with the help of electroencephalography (EEG), frontal electromyography (FEMG) and eye movement (EM) signals from the forehead of a patient's head. The invention also relates to a method of sensing pain reactions of a patient.
Electroencephalography (EEG) is a well-established method for assessing the brain function by picking up the weak signals generated in the brain with electrodes on the skull surface. To obtain the signals, multiple electrodes are placed on the scalp of a patient in accordance with a recognized protocol. EEG has been in wide use for decades in basic research of the neural system of brain as well as clinically in diagnosis of various neurophysiological disorders.
In a traditional EEG measurement electrodes are attached following the standard 10-20 system. Said system has been used by neurophysiologists for decades to record EEG and to find pathological EEG changes. The system however requires cumbersome attachment of multiple electrodes, especially when the electrodes are attached in the hair environment.
One of the special applications for EEG, which has received attention to during the 1990's is use of a processed EEG signal for objective quantification of the amount of brain activity for the purpose of determining the level of consciousness of a patient. In its simplest form, the usage of EEG allows for the automatic detection of the alertness of an individual, ie. if he or she is awake or asleep. This has become a significant issue, both scientifically and commercially, in the context of measuring the depth of unconsciousness induced by anesthesia during surgery. Modern anesthesia practices use a sophisticated balancing technique with a combination of drugs for maintaining adequate hypnosis, analgesia, muscle relaxation, and/or suppression of the autonomic nervous system and blockage of the neuromuscular junction. The need for a reliable system for monitoring of the adequacy of the anesthesia is based on both safety and economical concerns. An anesthesia dose, which is too light can, in the worst case, cause the patient to wake up during the operation and to create a highly traumatic experience both for the patient and for the personnel administering the anesthesia. At the opposite extreme, the administration of too deep anesthesia generates increased costs due to the excessive use of anesthesia drugs and the time needed to administer the drugs. Over dosage of the anesthetic drugs also affects the quality and length of the post-operative period immediately after the operation and the time required for any long-term post-operative care.
In the anesthesia and the intensive care said 10-20 system is cumbersome to use. This is because these environments are already crowded by many other measuring systems, such as blood pressure, ECG, inspired and expired gas measurements. The additional labour-consuming measuring system would take too much time and effort from the care personnel. There is even though need for central nervous system monitoring needs in these areas. The consciousness level of the patient is varied in both of said environments and till today there has not been a practical method for monitoring the level of consciousness in the anesthesia and the intensive care environment.
As told before in the anesthesia environment patient is anesthetized with hypnotic, analgesic and neuromuscular blocking agents. The neuromuscular blocking agents, given in a certain extent block the neuromuscular junction and the patient looses ability to move herself or himself. This can create a situation where patient feels pain but cannot communicate. Without central nervous system monitoring there is a risk of giving too little or too much anesthetics. If too little hypnotic drugs are given to the patient he or she could awake during operation, which could cause traumatic experience especially for the patient and also for the personnel.
Another important issue about the use of EEG is defining the level of sedation in Intensive Care Units (ICU). However, the situation in ICU is a little bit more complicated than in Operating Rooms (OR). In ICU patient is sedated by sedatives, which means that he or she is usually at the higher level of consciousness that in anesthesia. Sedatives are usually same medicines as anesthetics, but the doses are lighter. Sedatives have usually both hypnotic and analgesic components, but neuromuscular blocking agents are very rarely used in ICU. Without central nervous system monitoring patients are usually over sedated, which leads to longer treatment periods. Sedation induces amnesia, which afterwards often causes physiological problems to the patients. This is because of gap in memory.
Because neuromuscular blocking agents are not often used during sedation patients are able to move themselves when feeling pain. So patient might be agitated and he or she might be eg. fighting against ventilator, which cause huge amount of artifacts to the processed signal. If patient is not agitated he or she might be alert about surroundings, so patient can eg. move his or her eyes, which causes eye movement (EM) artifacts to the EEG. It is essentially important to identify those artifacts and reject contaminated signal periods from further EEG signal processing. Also patients in ICU might have neurological disorders, eg. status epilepticus, hydrocephalus, brain tumor or subarachnoidal hemorrage (SAH). This causes irrelevancies and non-symmetries to EEG. It is important to detect these phenomena without delay to ensure proper treatment of the patient.
The above mentioned reasons have generated commercial efforts to develop EEG devices to said environments during the past ten years. The main requirements for such monitoring can be described by the following features, ease of use, reliability and good quality. The efforts in this area have concentrated into reliable and easy-to-use electrodes as well as to good quality signal processing.
A significant main advancement in making EEG-based measurement of the adequacy of anesthesia or sedation an easy-to-use routine was a finding based on Positron Emission Tomography (PET) that determined that the effects of the anesthetic drugs on the brain are global in nature. This means that for many applications it is enough to measure the forebrain or frontal cortex EEG from the forehead of the patient. The forehead is both an easy to access and hairless location on the patient. Electrodes placed with an appropriate spacing between the electrodes on the forehead can pick up an adequate signal originating from the anterior cortex in the brain.
Since the Positron Emission Tomography (PET) studies have shown that the anesthesia effect is a global phenomenon in the brain, the sensor development efforts have concentrated on the hairless frontal area of the head. The first commercial sensor for this application area was developed by the company Aspect Medical Systems, Inc. U.S. Pat. No. 6,032,064 can be mentioned as an example of the art describing the sensor developed by Aspect Medical Systems, Inc. The company mentioned above also has patented many electrode configurations relating to placement of the electrodes on frontal and temple areas of the patient's head. Reference is made here to U.S. Pat. No. 6,394,953.
While the foregoing has discussed the use of EEG signals, it is also desirable to obtain frontal electromyographic (FEMG) signals arising from the forehead of the patient. The frontalis muscle is the first indicator of approaching consciousness. When this muscle activity is sensed by appropriately placed electrodes it provides an early indication that the patient is emerging from anesthesia. Similarly these electrodes can sense pain reactions when the anesthesia is not adequate, for example because of inadequate analgesia. So the FEMG signals give an early warning of arousal and may also indicate inadequate analgesia.
During sedation information from FEMG is essential to assess optimal level of sedation. In optimally sedated patients frontalis muscle responses to noxious stimuli, but if sedation is too deep frontalis muscle is unresponsive. To assess optimal level of sedation information about patient level of consciousness is also needed, meaning that simultaneous processing of EEG and FEMG signals must be performed. Because spectrums of EEG and FEMG signals overlap discrimination of signals requires sophisticated algorithms or optimal electrode position on the forehead.